The remarkable qualities of 2D layered materials such as wide spectral coverage, high strength and great flexibility mean that ultrathin 2D layered materials have the potential to meet the criteria of next-generation optoelectronic devices. Photoconductivity is one of the critical parameters of materials applied to optoelectronics. In contrast to traditional semiconductors, specific ultrathin 2D layers present anomalous negative photoconductivity. This opens a new avenue for designing novel optoelectronic devices. It is important to have a deep understanding of the fundamentals of this anomalous response, in order to design and optimize such devices. In this review, we provide an overview of the observation of negative photoconductivity in 2D layered materials including graphene, topological insulators and transitional metal dichalcogenides. We also summarize recent reports on investigations into the fundamental mechanism using ultrafast terahertz (THz) spectroscopies. Finally, we conclude the review by discussing the existing challenges and proposing the possible prospects of this direction of research.

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Photoluminescence (PL) and electroluminescence (EL) properties of a poly(p-phenylenevinylene) (PPV) derivative containing crown ether (C−PPV) were investigated. The PL quantum yield as high as 90% in solution and a temperature independence were observed. We infer that the surrounding side group, crown ether, effectively confines the excitons for radiative emission. The planar structure of crown ether also increases interchain interactions in the polymer and induces the formation of aggregation, which makes the PL spectrum of its solid film red-shifted. Through the Langmuir−Blodgett method, the evidence for aggregate formation is demonstrated. The polymer has been used in light emitting diodes. Using C−PPV as an emitting layer, the single layer device shows a low turn-on voltage (ca. 3 V), whereas the double layer device with a structure of ITO/C−PPV/tris(8-hydroxyquinolinato)aluminum (Alq3)/Al shows a moderate EL performance with a brightness of 730 cd/m2 and an external efficiency of 0.04%.